This invention relates to a loop antenna device.
A conventional loop antenna device is disclosed in German Patent Publication DE 4105826 or Japanese Patent Laid-Open Publication No. 2000-261245. The former conventional loop antenna device is shown in FIG. 8(a) and FIG. 8(b). As shown in FIG. 8(a) and FIG. 8(b), this loop antenna device 51 includes a first antenna 55 and a second antenna 58. The first antenna 55 has a coil 53 wound around a ferrite rod 52 and a resonant capacitor 54 connected to the coil 53 and constitutes a parallel resonant circuit. The second coil 58 has a circular coil 56 magnetically connected to the coil 53 and a resonant capacitor 57 connected to the circular coil 56 and constitutes a parallel resonant circuit.
When a high frequency is fed to a coil 59 wound around the ferrite rod 52 from a power source 60, a magnetic field component is generated by the first antenna 55 in the y-axis direction and a magnetic field component is generated by the second antenna 58 in the z-axis direction. Thereby, a composite magnetic field is generated in the y-z-axis direction and a predetermined electric wave corresponding this composite magnetic field is radiated from the loop antenna device 51 when each resonant circuits of the first and second antennas 55 and 58 resonated.
Furthermore, the latter conventional loop antenna device is shown in FIG. 9. As shown in FIG. 9, in this loop antenna, a request signal output circuit 62 which constitutes a transmitter of an antenna 61 includes a crystal oscillator 63, an oscillating circuit 64, a D-type flip-flop 65, two amplification circuits 66, 67 and a modulation circuit 68. The output terminals of the amplification circuits 66, 67 are connected to magnetic field generating parts (coils) 69, 70 which are disposed while declining with 90 degree each other, respectively. Resonant capacitors 71, 72 are connected to the coils 69, 70, respectively and a resonant circuit is constituted by the coils 69, 70 and the resonant capacitors 71, 72, respectively.
A predetermined pulse signal which is outputted from an output terminal Q1 of the oscillating circuit 64 is fed to the coil 69. A pulse signal whose phase is shifted with 90 degree with respect to the pulse signal from the output terminal Q1 is fed to the coil 70 by the flip-flop 65. Thereby, a composite magnetic field (a rotational magnetic field) which has directional characteristics of 360 degree is generated by the coils 69, 70 and a predetermined electric wave corresponding this composite magnetic field is radiated from the antenna 61 in response to a timing of a control signal outputted from a microcomputer 73.
In the former loop antenna device 51 shown in FIG. 8, however, although the first antenna 55 is disposed inside of the circular coil 56, since empty space is large, the size of the antenna device increases. On the other hand, in the latter conventional antenna 61, the electric wave continues radiated due to a discharge phenomenon of the resonant capacitors 71, 72 after the output of the pulse signal is ended. Namely, as shown a wave form of an antenna output in FIG. 10, the energy stored in the resonant capacitors 71, 72 is discharged for a T interval after the output of the pulse signal is ended and the electric wave from the antenna 61 continues radiated. Accordingly, in case that a next data is sent after a certain data is placed on the electric wave and is sent, it is necessary to set a time margin until the end of the discharge of the resonant capacitors 71, 72. As a result, it is impossible to increase a data sending speed.
In order to overcome the drawback regarding the data sending speed, for example, it is found to be useful that a damping resistance is connected to the resonant circuit. When a damping is performed by the damping resistance, however, the damping is always performed to the resonant circuit independently of with or without the radiation of the electric wave and the extra energy is consumed. Namely, the energy on the resonant circuit is always consumed by the damping resistance. Thereby, an antenna gain or a radiant efficiency which affect a transceiving (receiving and sending) sensitivity of the electric wave decrease and high input has to be given to the resonant circuit for preventing the decrease of the antenna gain or the radiant efficiency.
A first object of the present invention is to provide a loop antenna device which inhibits the unnecessary radiation of the electric wave and which can increase the sending speed of data placed on the electric wave. A second object of the present invention is to provide a loop antenna device which can achieve the first object and which can perform the damping to the resonant circuit without influencing the antenna gain or the radiant efficiency greatly.
The invention according to one aspect provides a loop antenna device comprising;
a first loop antenna constituting a resonant circuit by a coil and a resonant capacitor and resonating on the basis of a high frequency signal of a resonant frequency intermittently outputted from an oscillation means; a second loop antenna constituting a resonant circuit by a coil and a resonant capacitor and resonating by an inductive electromotive force led by a mutual induction via a link coil when the first antenna resonates; and a damping means for compulsory eliminating a discharge phenomenon of the resonant capacitor when the radiation of an electric wave is completed and connected to at least one of the first loop antenna and the second loop antenna.
When the first loop antenna resonates by the high frequency signal of the resonant frequency, the second loop antenna resonates by the mutual induction via the link coil and an electric wave is radiated from the loop antenna device. When the radiation of the electric wave is completed, the electric charge stored in the resonant capacitor of the resonant circuit is discharged and a discharge phenomenon generates. However, since this stored energy is consumed as a heat energy by the damping means and the discharge phenomenon is compulsory eliminated, the unnecessary radiation of the electric wave is inhibited. Thereby, it is unnecessary to set a time margin until the unnecessary radiation of the electric wave is completed and the sending speed of data placed on the electric wave can be increased.
The damping means is a switching means whose ON-OFF condition is changed by a digital control signal outputted from a control means, and the resonant circuit constitutes a closed circuit when the switching means is in the ON condition on the basis of the control signal, and the discharge phenomenon of the resonant capacitor is compulsory eliminated by the switching means when the switching means is in the OFF condition in response to the change of the level of the control signal.
When the switching means becomes the ON condition on the basis of the control signal outputted from the control means, the resonant circuit constitutes a closed circuit and resonates. When the level of the control signal is changed and the radiation of the electric wave is completed, the switching means becomes the OFF condition and an internal resistance is generated in the switching means. The electric charge stored in the resonant capacitor is consumed at once as a heat energy by the internal resistance. Thereby, the unnecessary radiation of the electric wave is inhibited.
The switching means are provided on both of the first and second loop antennas and are changed between the ON-OFF condition by the same control signal.
The discharge phenomenon generated in both of the resonant capacitors of the first and second loop antennas is compulsorily eliminated and the reliability of the loop antenna device is improved.
The invention according to another aspect provides a loop antenna device comprising; a first loop antenna constituting a resonant circuit by a coil and a resonant capacitor and resonating on the basis of a high frequency signal of a resonant frequency intermittently outputted from an oscillation means; a second loop antenna constituting a resonant circuit by a coil and a resonant capacitor and resonating by an inductive electromotive force led by a mutual induction via a link coil when the first antenna resonates; and a damping means connected to the second loop antenna; wherein a connecting condition of the damping means is changed in response to a timing of a high frequency signal outputted from the oscillation means which is connected to the first loop antenna, and the damping means makes the resonant circuit of the second loop antenna in a connected condition when the high frequency signal is in output condition, and the damping means eliminates compulsory a discharge phenomenon of the resonant capacitor when the high frequency signal is in non-output condition.
When the high frequency signal of the resonant frequency is outputted from the oscillation means, the high frequency signal is outputted to the damping means and the condition of the resonant circuit of the second loop antenna becomes the connecting condition. In this condition, when the resonant circuit of the first loop antenna resonates by the high frequency signal, the second loop antenna resonates by a mutual induction via the link coil and an electric wave is radiated from the loop antenna device. When the high frequency signal is not outputted and the radiation of the electric wave is completed, the electric charge stored in the resonant capacitor of the second loop antenna is discharged and a discharge phenomenon generates. However, since this stored energy is consumed as heat energy by the damping means and the discharge phenomenon is compulsory eliminated, the unnecessary radiation of the electric wave is inhibited. Thereby, it is unnecessary to set a time margin until the unnecessary radiation of the electric wave is completed and the sending speed of data placed on the electric wave can be increased.
The oscillation means has two switching means which are connected in series between an electric power source and a ground, and outputs the high frequency by the changing of the ON-OFF condition of the switching means by a control means, and one of the switching means connected to the ground functions also as the damping means of the first loop antenna.
When the radiation of the electric wave is completed, the electric charge stored in the resonant capacitor of the first loop antenna is discharged and a discharge phenomenon is generated. However, since this stored energy is consumed as heat energy by the damping means and the discharge phenomenon is compulsorily eliminated, unnecessary radiation of the electric wave is inhibited. Thereby, since the discharge phenomenon generated in both of the resonant capacitors of the first and second loop antennas is compulsorily eliminated and the reliability of the loop antenna device is improved. Furthermore, since the switching means of the oscillation means functions also as the damping means which eliminates compulsory the discharge phenomenon of the resonant capacitor of the first loop antenna, it is able to reduce the number of parts of the loop antenna device.
One of the switching means of the first loop antenna makes the resonant circuit of the first loop antenna be a closed circuit when one of the switching means become the ON condition, and one of the switching means of the first loop antenna eliminates compulsorily the discharge phenomenon of the resonant capacitor when one of the switching means become the OFF condition.
When the radiation of the electric wave is completed, the switching means becomes the OFF condition and an internal resistance is generated in the switching means. The electric charge stored in the resonant capacitor is consumed at once as a heat energy by the internal resistance. Thereby, unnecessary radiation of the electric wave is inhibited.
The damping means of the second loop antenna is a switching means whose ON-OFF condition is switched by a control signal converted the high frequency signal, and the resonant circuit of the second loop antenna constitutes a closed circuit when the switching means becomes the ON condition on the basis of the control signal, and the discharge phenomenon of the resonant capacitor is compulsory eliminated by the switching means when the switching means becomes the OFF condition by the change of the level of the control signal.
When the high frequency signal is outputted and the switching means becomes the ON condition, the resonant circuit of the second loop antenna constitutes a closed circuit and resonates. When the level of the control signal is changed and the radiation of the electric wave is completed, the switching means becomes the OFF condition and an internal resistance is generated in the switching means. The electric charge stored in the resonant capacitor of the second loop antenna is consumed at once as heat energy by the internal resistance. Thereby, unnecessary radiation of the electric wave is inhibited.
A signal converting means is also provided for converting the high frequency signal outputted from the oscillator means into a digital control signal which switches the operating condition of the damping means of the second loop antenna and is connected between the damping means of the second loop antenna and the first loop antenna.
The connecting condition of the damping means of the second loop antenna is changed by the control signal converted by the signal converting means.
The signal converting means includes a smoothing means for smoothing the high frequency signal outputted from the oscillator means and a demodulation means for converting the converted signal smoothed by the smoothing means into a control signal for switching the connecting condition of the damping means of the second loop antenna.
The high frequency signal outputted from the oscillation means is smoothed by the smoothing means. The converted signal smoothed by the smoothing means is converted into a control signal for switching the connecting condition of the damping means of the second loop antenna by the demodulation means.
The resonant circuit of the first loop antenna and the resonant circuit of the second loop antenna are constituted by one of the series resonant circuit and the parallel resonant circuit.
The resonant circuit of the first loop antenna and the resonant circuit of the second loop antenna are constituted by one of the series resonant circuit and the parallel resonant circuit.